Pretty simple: brake inspection showed all four brakes good (brake pads / rotors / calipers). Brake master cylinder full of brake fluid and not brake fluid leaks in brake hydraulic system. Brake pedal very firm with very little pedal-travel. Test-drive (CAUTIOUSLY) demonstrated basic brakes working but NO power assist to brakes. Note that this vehicle uses a brake VACUUM power assist booster – a very traditional and wide-spread power brake system. Tracing the vacuum source line revealed a broken plastic fitting on the vacuum pump:

LR4 broken brake booster vacuum pump fitting – in engine bay

Further inspection showed that one of the engine drive belts had frayed and a sliver of it had broken off and (presumably) hit this plastic fitting and broken it – causing the vehicle to have a sudden loss of brake power assist.

LR4 – Jaguar AJ V8 broken brake vacuum pump

Land Rover broken vacuum pump output fitting – plastic fitting

NOTE – KEY to the problem is the location of the water pump belt in relationship to this vacuum pump’s fitting:

Land Rover V8 5.0L vacuum pump fitting near belt

The drive belt that you see in the picture is the water pump and cooling fan belt. It is of more modern design in that it is a ‘stretch to fit’ belt – it has no tensioner assembly – it is self tensioning. Our opinion is that this belt failed and hit the vacuum pump fitting and broke it – causing sudden braking power loss.

Repair Process:

The repair was straight-forward: replace the belt and the vacuum pump (the fitting is NOT available as a separate service part from Land Rover). Interestingly the vacuum pump was on national back-order through Land Rover USA and not available through aftermarket suppliers. Land Rover updated the vacuum pump part number (this usually means an update to the design) however when we received the replacement pump we were disappointed to see it still has a plastic fitting for the vacuum output.

LR4 new vacuum pump

LR4 new vacuum pump – front view

Land Rover V8 updated vacuum pump – plastic output fitting

Here’s the special tool used to help install the stretch belt. We suspect that the installation process can create abnormal strain and possibly premature wear/failure of the belt. This tool makes the job easier and reduces that risk.

Land Rover V8 5.0 stretch belt installation

Here’s the new pump installed – NOTICE the proximity to the water pump belt:

New Vacuum Pump Installed

Technical Discussion:

Traditionally a brake vacuum booster is provided its vacuum by a large vacuum hose attached to the engine intake with a one-way check valve to preserve the vacuum. Some engines – notably Diesels – don’t produce vacuum and thus the design has to change to either an alternative vacuum boost or other power source. Some manufacturers have elected to use hydraulic pressures for this type of system with the most common being General Motor’s Hydraboost system – which taps power from the power steering pump. Other manufactures continue to use vacuum as the brake booster power source and add an auxiliary pump to produce the needed vacuum. As brake vacuum pumps were introduced the typical setup was to provide the brake vacuum boost system with two vacuum sources: a pump like this on AND engine vacuum. This Land Rover setup uses a vacuum pump exclusively and, unfortunately, the physical location and the decision to use plastics makes this engine and its vehicles susceptible to loss of brake power assist. Our advice to vehicle owners: be sure to service your belts before they fail. Our advice to the vehicle engineers out there: if you MUST use a design like this have a belt guard or more substantial materials to ensure a belt failure cannot cause braking power loss.

Here’s something we see too frequently: brake hoses that are cracking. These were on an Audi, which, for some strange reason, seems to have this issue more frequently. The fix, obviously, is to replace the lines. This is a potentially life-threatening problem so don’t take a chances on this one: